The aim of this work is to determine the DNA sequence organization of class I genes contained in the swine major histocompatibility complex (SLA). We have established that this gene family contains only seven members, of which five have been isolated and their DNA sequences determined. It has been established that the sequence organization of the class I SLA genes is similar to that of other class I genes in other species. Within the family, it is possible to define at least three sub-families, based on their sequences. Homologies between the SLA genes range from 85% between two genes within a sub-family to 45% between subfamilies. Gene conversion events appear to give rise to complex alterations, and to occur between sub-families. Physical mapping has established that the class I genes map within 500 kb on a single linkage group. Isolation of cDNA clones has demonstrated that only 4 class I genes are expressed within swine PBL populations. Sequence analysis of these clones has established the locations of exons and further indicates a variety of post-transcriptional alternative splicing events occurs in these cells.